1. Field of the Invention
The present invention relates to an expanding apparatus for expanding an agricultural product such as a tobacco material, or food. More particularly, the present invention relates to a continuous type expanding apparatus using gaseous carbon dioxide as an expanding agent, which has a cooling unit capable of reliably controlling at a low temperature the material transported from an impregnating vessel to a material discharge system and efficiently expanding the material.
2. Description of the Related Art
According to some conventional expanding apparatuses, the material, e.g., a tobacco material is impregnated with carbon dioxide as an expanding agent at a high pressure, and the tobacco material is pressure-decreased and heated, so that the impregnated carbon dioxide is expanded, thereby expanding the tobacco material.
The expanding apparatuses are classified into batch type expanding apparatuses and continuous type expanding apparatuses. In a batch type expanding apparatus, a predetermined amount of tobacco material is stored in an impregnating vessel, high-pressure carbon dioxide is supplied to the impregnating vessel to impregnate the tobacco material with carbon dioxide, and thereafter the tobacco material is removed, thereby expanding the tobacco material. In a continuous type expanding apparatus, the tobacco material and carbon dioxide are continuously supplied to an impregnating vessel.
Although the former batch type apparatus has a simple structure, its efficiency is low and a large amount of carbon dioxide is lost. The latter continuous type expanding apparatus is efficient and can recover and re-utilize carbon dioxide.
In order to generally increase the expansion degree of the tobacco material or the like, the tobacco material must be brought into contact with carbon dioxide at a low temperature and a high pressure so that the material is impregnated with a maximum amount of carbon dioxide. The tobacco material impregnated with carbon dioxide must be removed from the impregnating vessel while maintaining the low temperature as much as possible, loss of impregnated carbon dioxide must be prevented, and the tobacco material must be heated instantaneously, thereby effectively expanding the impregnated carbon dioxide.
However, in the continuous type apparatus described above, the temperature and supply amount of the tobacco material supplied to the impregnating vessel, the quantity of external heat applied to this expanding apparatus, the quantity of frictional heat generated when the rotary valve is rotated, and the like vary over a considerably large range. Therefore, because of these variations in conditions, the temperature of the tobacco material supplied to the impregnating vessel is increased to decrease the impregnation amount of carbon dioxide, or the tobacco material removed from the impregnating vessel is heated while it passes through the rotary valve, and part of the impregnated carbon dioxide is lost, thereby decreasing the expansion degree.
In order to prevent these drawbacks, it is considered to cool and, if necessary, partly liquefy carbon dioxide to be supplied to the impregnating vessel in order to absorb heat generated in the material or in the components in the downstream of the impregnating vessel by the latent heat and sensible heat of carbon dioxide, thereby maintaining the material at a low temperature. However, if the cooling amount of carbon dioxide, i.e., the heat quantity to be removed is excessively small, the tobacco material or the components in the downstream of the impregnating vessel are not sufficiently cooled, not providing much effect. Inversely, if the cooling amount of carbon dioxide is excessively large, carbon dioxide is solidified to form dry ice while the tobacco material is pressure-decreased and discharged from its discharge system. When dry ice is formed in this manner, the tobacco material is solidified by it, causing a problem in the heating/expanding step. Furthermore, the amount of carbon dioxide discharged to the outside of the system together with the material is also increased, leading to an increase in loss of carbon dioxide. Such an operation to produce dry ice is not preferable in terms of economy and quality. Therefore, carbon dioxide must be impregnated in the impregnating vessel in a gaseous state. For this purpose, the cooling amount (heat exchange amount) of carbon dioxide to be supplied to the impregnating vessel must be appropriately controlled.
However, the temperature of tobacco material, the supply amount of tobacco material, the amount of external heat applied to the expanding apparatus, the heat quantity of the rotary valve, and the like are not stable and vary over a wide range. For this reason, it is difficult to impregnate the tobacco material with gaseous carbon dioxide with a preferable condition in the impregnating vessel.
In addition, control of the cooling amount (heat exchange amount) of carbon dioxide described above is generally considered to be performed by controlling the amount or temperature of carbon dioxide to be supplied. However, since the amount of carbon dioxide is determined to maintain the impregnating pressure in the impregnating vessel at a predetermined value, the above control cannot be performed. Regarding the temperature, since carbon dioxide is subjected to phase transition depending on the pressure and temperature, the temperature cannot be employed as a control factor. Accordingly, the cooling amount of carbon dioxide cannot be controlled by the amount or temperature of carbon dioxide.